This invention relates generally to apparatus and methods for balancing electrical currents in a multiple-phase electrical system. The invention relates more particularly, but not by way of limitation, to an apparatus and a method for balancing any of the phase currents in a three-phase electrical system in response to a direct current which is proportional to the average of the three alternating phase currents.
In a three-phase electrical power system, for example, to which a three-phase motor is connected for energization, many problems can arise which degrade the system and the operation and life of the motor. For example, when the motor is started, significant electrical and mechanical stresses are imposed on the motor because of the high starting currents normally used to start the motor. For example, there are electric submersible motors which have a starting current of six to eight times the nominal rating. Such high starting currents cause the system voltage to sag and thereby affect not only the motor, but also other electronic circuits on the system.
Even after a motor is started, significant stresses can be applied to the motor due to unbalanced operating currents. These unbalanced currents can cause excessive heating and increased power consumption. Also during operation, electrical transients can occur in the system, such as from lightning or switching surges. These undesirable operating conditions adversely affect not only the electrical load, but also the overall system.
The foregoing illustrates the need for an apparatus which eliminates or reduces electrical system current imbalances. There is also the need for an apparatus to eliminate or reduce electrical transients. There is the further need for an apparatus which allows the soft-starting of electrical loads, such as motors, to reduce excessive stresses imposed upon such loads during start-up. If these needs were met, the operating life and dependability of electrical loads could be increased and power consumption could be reduced.
An apparatus which meets such needs should be electrically and mechanically dependable and efficient to enhance the structural, operational and economic features of such an apparatus. Such an apparatus should also be capable of being installed and removed from the power system without causing expensive downtime of the system. Such an apparatus should also be capable of use without additional step-up transformers and with a minimum of special training for installation and maintenance.
Although the aforementioned needs can be met by my earlier invention disclosed in U.S. Pat. No. 4,574,231, there is the further need for an improved current balancing apparatus and method which utilize only one control current regardless of the number of phases of alternating current to be controlled because this would make the control circuit relatively simple and inexpensive to manufacture. There is the still further need for the control current to be a direct current which has a percent ripple substantially lower than other direct current control currents derived from less than all of the phases within the multiple-phase system because having a relatively lower percent ripple would produce lower power losses and permit the use of at least some relatively smaller, less expensive components.